Hit a horse stable for your free manure. Just takes spending some sweat equity and buying some Preen for any stray seeds. There isn't anything in that that won't decompose and make you ill.
I've heard horse manure is the best, and I use our donkey;s manure and it's fabulous. But you do want to know what the horses have been eating. A local gardener here got some free horse manure and unfortunately the horses had been eating or grazing hay from a field that had been sprayed with herbicide. Everything he planted where he used the manure died, and nothing would grow for two years.
Yes, herbicide problems seem to be rampant. People use them to control broad leaf weeds in pastures and hay fields; the chemicals pass through and are very long lasting. If the manure has been composted, you can try sprouting pea seeds in it (they are very sensitive to contamination) to test the material. However, if you aren't careful and do contaminate your garden it can take years to clear.
Yes to what both PhilaGardener and flowerweaver said. Picloram is a herbicide that can remain in the plant matter for years and can pass through an animal's digestive system still potent enough to kill or stunt new plant growth for several years after use.
Legumes are particularly vulnerable to Picloram's effects so planting peas in soil containing your animal manure or compost will be a quick test if Picloram was used.
But not in pastures correct? My (maybe erroneous ) understanding is that it's effectiveness on pasture weeds means it can be applied with no withdrawal period for livestock, and the herbicide passes through them without harm to them but takes two years to completely break down in the manure.